Trace metal uptake in vivianite [Fe₃(PO₄)₂.8H₂O]: experimental and computational studies

Principal Supervisor: Dr. Karen Hudson-Edwards (Birkbeck Earth Sciences)
Co-Supervisors: Dr. Kevin Taylor (Manchester Metropolitan University) & Dr Maria Alfredsson (UCL Earth Sciences)

The cycling of contaminant elements and nutrients between freshwater lake sediments and waters plays a key role in the chemical and ecological functioning of these important environments. It has long been known that early diagenesis plays a particularly strong role in the short- to long-term release of iron, manganese and phosphorus. For example, the bacterial reduction of iron(III) and manganese(IV) oxides leads to phosphorus and contaminant release, and subsequent diffusive flux to the overlying water column. In marine aquatic sediments the precipitation of highly stable sulfide minerals (e.g. FeS2), as a consequence of early diagenetic bacterial sulfate reduction, has been clearly documented to act in the reverse manner by acting as a highly effective, chemically stable sink for Fe and metallic and metalloid elements (e.g. copper, zinc, cadmium, arsenic). The absence of significant sulfate reduction in freshwaters has led to the assumption that such sinks do not exist in freshwater sediments. It is known, however, that the iron phosphate vivianite [Fe3(PO4)2.8H2O] may precipitate during early diagenesis in freshwater sediments, and it has recently been documented that vivianite can be abundant in nutrient-rich freshwater sediments. Vivianite is the Fe-rich end-member of the vivianite mineral group [X3(YO4)2•8H2O, where X=Co, Fe, Mg, Mn, Ni or Zn, and Y=P or As]. This general formula suggests that vivianite should be able to incorporate substantial amounts of the potentially toxic elements zinc and arsenic, and this is confirmed by preliminary research by one of the proposed supervisors that suggests that freshwater vivianites from the Manchester Ship Canada, UK, contain thousands of ppm of zinc. It should be therefore theoretically possible for vivianite to sorb or incorporate (into the ‘X’ site) other 2+ metallic ions (such as copper and lead) that exist, sometimes in excessive quantities, in freshwater systems. The proposed supervisors are currently investigating the incorporation of lead into natural vivianites from the Manchester Ship Canal, but further research is required, using ‘ideal’, well-characterised, synthetic vivianites, on the incorporation of both lead, zinc and copper (another common, potentially toxic metallic element). If these elements are co-precipitated with, or sorbed to, vivianite, this has important implications for the role of this mineral in the cycling of contaminants in freshwater systems.

The aim of this research is to test the hypothesis that synthetic vivianite acts as a significant sink for lead, zinc and copper. This aim will be fulfilled by carrying out the following specific objectives:
(i) synthesising and fully characterising pure iron end-member vivianite;
(ii) carrying out batch experiments using the material from (i) to determine if, and to what extent, lead, zinc and copper are sorbed onto the vivianite;
(iii) synthesising and fully characterising iron-lead, iron-zinc and iron-copper vivianite (i.e., co-precipitating lead, zinc and copper with iron in vivianite), and determining the maximum amount of lead, zinc and copper that can be incorporated into the vivianite structure;
(iv) carrying out computational modelling of the vivianite structure to determine the most suitable sites for lead, zinc and copper incorporation.

The studentship will provide training in environmental geochemistry and mineralogy, and computational modelling. The student will have the opportunity to work in a dynamic environmental research environment in both London and Manchester.

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